The present invention relates to improved oxygen ion conducting materials, useful in, for example, ceramic electrolyte devices. More particularly, the invention relates to a class of materials that maintains the excellent conductivity and catalytic properties of undoped lanthanum manganese oxide or of an A-site doped lanthanum manganite perovskite material, while increasing oxide ion conductivity in the material. This increase is realized by a significantly lower overpotential and better performance compared to other unsubstituted or A-site substituted materials.
Ceramic electrolyte devices have a myriad of commercial uses in many devices that require oxygen anion conductivity, ranging from power generation in the form of solid oxide fuel cells to oxygen pumps, oxygen sensors, and air separation membranes. A ceramic electrolyte device comprises three separate parts: (1) a cathode that reduces elemental oxygen to oxide ions, (2) an electrolyte that transports the oxide ions, but not electrons, across a cell to an anode, and (3) the anode, where the oxide ions react with protons to form water. Currently, the material most commonly used for the cathode for these electrolyte devices is a doped lanthanum manganite, (La,A)MnO3, a perovskite oxide, where A=Ca or Sr, wherein only the lanthanum position is doped. In this class of materials, where oxide examples are denoted as ABO3, the large A-cation is typically a lanthanide, alkaline earth metal, or alkali metal cation in 7-12 coordination to oxygen. The B-cation is typically a transition or main group metal in octahedral coordination to oxygen. The compound LaMnO3 has advantages in two of the three main requirements for a ceramic electrolyte device material: good electrical conductivity and catalytic activity for oxygen reduction; however, it exhibits poor oxygen ion conductivity. The poor oxygen ion conductivity problem can be partly solved by maximizing the number of triple point boundaries in the cathode, but this solution requires careful manipulation of particle sizes and complex fabrication of the ceramic-electrolyte interface.
For LaMnO3 based ceramic electrolyte devices, one of the main limitations to the technology is that overpotential in the system is too high, causing unnecessary energy loss and inefficiencies. Presently when used in solid oxide fuel cells, typical LaMnO3 based cathodes have an overpotential of roughly 60 mV. For the cell to run more efficiently, the overpotential must be lowered. The overpotential can be lowered by increasing the oxygen ion conductivity of the cathode.
A generally accepted method for introducing oxide ion conductivity into ceramic oxides is to substitute a lower valent element for the principle cation. For example, in zirconia, ZrO2, vacancies can be introduced by addition of yttria (Y2O3) or calcium oxide (CaO), to form, for example, Zr1xe2x88x92xYxO2xe2x88x92x/2. In these instances, charge compensation is oxygen loss rather than reduction of zirconium cations. In other systems, such as LixNi1xe2x88x92xO, an effect of substituting lithium cations for nickel is oxidation of the nickel cations, rather than oxygen loss. It is a balance between these two separate equilibriums that is a key factor in increasing utility of the LaMnO3 system for ceramic electrolyte devices. Even in doped LaMnO3 systems, such as (La1xe2x88x92xSrx)MnO3, a preponderance of the doping is compensated for by oxidation of the manganese cations rather than oxygen loss. A 1989 study by Kuo, Anderson, and Sparlin [J. Solid State Chem. 83, 52-60 (1989)] on the effect of oxygen partial pressure on a charge compensation mechanism for a doped material (La0.80Sr0.20)MnO3 showed that manganese oxidation was favored in all cases where pO2 was greater than 10xe2x88x9212 atm at 1000xc2x0 C. and 10xe2x88x9210 atm at 1200xc2x0 C. In either case, no significant oxide ion vacancy formation was observed. These findings are significant because most ceramic electrolyte cathodes operate under conditions using a pO2 range of 10xe2x88x922-10xe2x88x923 atm, well within the oxygen stoichiometric regime for doped LaMnO3 materials. There is still a need, therefore, to develop new ceramic electrolyte electrode materials that maintain excellent conductivity and catalytic properties of LaMnO3 cathode materials while increasing their intrinsic oxide ion conductivity under realistic oxygen concentrations.
It is therefore an object of the present invention to provide novel oxygen ion conducting materials and compositions.
It is another object of the present invention to provide a novel lanthanum manganite material that maintains excellent conductivity and catalytic properties, while also increasing oxide ion conductivity in the material.
It is another object of the invention to provide a novel method for decreasing overpotential of electrode materials.
Other objects and advantages of the invention will become apparent by review of the detailed description of preferred embodiments.
The present invention is directed to an adjustment to a composition of lanthanum manganite electrolytes in order to enhance conductivity of oxygen ions. Lanthanum manganites are commonly used as electrodes in solid oxide fuel cells, oxygen pumps and air separation devices. They are doped with strontium or calcium and have good electrical conductivity and catalytic activity but poor oxygen ion conductivity, manifested in the form of a high overpotential in fuel cells. This invention is generally concerned with substitution of a lower or equal valent element for the principal cation which will increase the oxygen ion conductivity through creation of oxygen vacancies in the perovskite lattice. By increasing the oxygen ion conductivity of lanthanum manganite, energy efficiency of the system is increased, as is the material""s potential utility at different temperatures.
The above described objects and embodiments are set forth in the following description and illustrated in the drawings described hereinbelow.